A fake anti virus program may first appear as a legitimate looking pop-up window that tells the user that they have a dangerous virus. They are then prompted to click to buy anti virus protection to sort out their system. There is, however, no virus at all but the advert contains a potential problem if the user then clicks on it.
Clicking may then simply download malware on to the system. This may install malicious code that disables/damages the computer or that steals personal data. Or, as is often the case, the user will go through to a site where they enter credit card details to buy the supposedly legitimate anti virus software. Their financial details may then be stolen by the scammers and they may still be left with residual malware on their machines.